You can harvest yeast from the primary or the secondary.
BUT ...... by harvesting from the secondary you run the risk of getting mutations quicker. That is, the yeast characteristic will change quicker. By harvesting from the secondary, you are selecting the yeast that doesn't flocculate, as opposed to the yeast that did the job and then flocculated.
Sure, use the yeast from the secondary for another beer (maybe 2), but if your intention is to continue the line and get 5, or maybe 10, more batches from it, you really need to use the yeast from the primary if you expect to retain the original yeast characteristics.